The basic components of electrical capacitors include conductive electrodes connected to an electric power supply and a dielectric material separating the electrodes. Electrolytic capacitors and electrochemical double layer capacitors also have an electrolyte. In an electrolytic capacitor, the electrodes are provided by an oxide or carbon layer formed on metal foil and separated by a porous non conducting membrane such as paper, porous propylene, etc. The liquid electrolyte provides electrical contact to the opposite electrode through the separator. The inherently high resistance of electrolytic capacitors is generally mitigated by rolling a large sheet of the electrode material into a roll to give high surface area. In an electrochemical double layer capacitor, the dielectric is provided by the electrolyte. In this type of capacitor, the resistance of the electrolyte is a significant factor in the total device resistance. In capacitors that use electrolytes, the temperature has a major influence on the electrolyte in the performance of the capacitor since the conductivity of the electrolyte decreases with temperature.
Electrochemical double layer capacitors, including super capacitors, typically comprise electrodes, electrical contacts to a power supply, separators for electrodes and/or cells, an electrolyte and environmental seals. As mentioned above, a key component of electrolytic and electrochemical double layer capacitors is the electrolyte, which typically comprises a combination of a conductive salt and a solvent. Desirable electrolytes are typically liquid with low viscosity, low density, and high conductivity over a range of ambient temperature conditions. They should also be commercially inexpensive, chemically and electrochemically stable, and compatible with carbon. Aqueous electrolyte systems have been used extensively and provide voltage below 1.8v. However, certain organic aprotic liquid systems are less prone to form gas and can be more effective in providing higher energy densities over a wider usable range of temperature and potential. In addition, these organic electrolytes permit higher voltage and therefore results in higher capacity in the capacitors. The current non-aqueous aprotic solvent used for ultra capacitor electrolytes is acetonitrile which is toxic, highly flammable and has a voltage limit of 2.8v. For example, ultra capacitors in Japan are not permitted to use acetonitrile for the electrolyte. A need exists for improved electrolyte systems that provide optimum capacitance for capacitors to achieve high power density, a wide temperature range, and a long lifetime without memory effects.
The key requirements for the electrolyte in both non-aqueous batteries and capacitors are high voltage stability, low temperature performance and electrochemical stability.
U.S. Pat. No. 5,418,682 to Warren et al, which is herein incorporated by reference discloses a method of preparing tetraalkyl ammonium tetrafluoroborate salts for use as electrolytes with dinitrile mixtures as solvents.
U.S. Pat. No. 5,965,054 to McEwen et al, which is herein incorporated by reference discloses non-aqueous electrolytes for electrical storage devices utilizing salts consisting of alkyl substituted, cyclic delocalized aromatic cations and their perfluoro derivatives with alkyl carbonate solvents.
U.S. Pat. Nos. 6,535,373 and 6,902,684 to Smith et al, which are herein incorporated by reference, disclose similar electrolytes which utilize nitrile solvents.